Brake warning switch requiring recentering pressure

ABSTRACT

A brake warning switch or other differential pressure sensing device has opposed pressure chambers in a housing to be connected to the fluid circuits of a dual brake system and includes a pressure responsive piston assembly shiftable from a centered position in the housing to generate a warning signal upon an abnormal pressure imbalance in the chambers. The piston assembly is arranged to automatically recenter itself when pressures are reestablished in the chambers and includes a pair of coaxial pistons of differing areas and associated ring seals at one chamber, at least, arranged to permit the differential area of the pistons, which determines the relative magnitude of the pressures necessary to recenter the pistons, to be selected so that the recentering pressure required is high enough to test the integrity of the fluid circuits.

United States Patent [191 Orzel BRAKE WARNING SWITCH REQUIRINGRECENTERING PRESSURE Edward S. Orzel, Cleveland, Ohio The WeatherheadCompany, Cleveland, Ohio Filed: July 26, 1971 Appl. N0.: 166,082

Inventor:

Assignee:

References Cited UNITED STATES PATENTS 11/1970 Falk 340/52 C 12/1971 Tam340/52 C 1/1972 Harperm, 340/52 C 8/1918 73/419X Gilling Jan. 1, 1974Primary Examiner--Richard C. Queisser Assistant ExaminerDaniel M. YasichAttorney-McNenny, Farrington, Pearne & Gordon [5 7] ABSTRACT A brakewarning switch or other differential pressure sensing device has opposedpressure chambers in a housing to be connected to the fluid circuits ofa dual brake system and includes a pressure responsive piston assemblyshiftable from a centered position in the housing to generate a warningsignal upon an abnormal pressure imbalance in the chambers. The pistonassembly is arranged to automatically recenter itself when pressures arereestablished in the chambers and includes a pair of coaxial pistons ofdiffering areas and associated ring seals at one chamber, at least,arranged to permit the differential area of the pistons, whichdetermines the relative magnitude of the pressures necessary to recenterthe pistons, to be selected so that the recentering pressure required ishigh enough to test the integrity of the fluid circuits.

7 Claims, 2 Drawing Figures BRAKE WARNING SWITCH REQUIRING RECENTERINGPRESSURE BACKGROUND OF THE INVENTION the risk of a total brake failureand a resulting high risk v to life and property.

With a failure in one of the dual circuits, the remaining circuit isstill capable of slowing and stopping the vehicle. The braking actionprovided by only one operable circuit may be sufflciently effectiveunder normal braking situations that the operator may not become awareof the failure in one circuit. The full braking capacity of the vehicle,though, would no longer be available in an emergency. Moreover,subsequent failure of the remaining operable circuit would leave thevehicle without braking capability which is the condition that thed'ualsystem is intended to eliminate. For these reasons it is importantto provide apparatus to warn the vehicle operator of a failure in one ofthe brake circuits.

One particular problem has been that prior art devices have generallybeen of two types. One type has had the switch actuating member springcentered so that it produces a warning signal only when the brakes areapplied and a pressure differential exists. In the other type theactuating member remains in the actuate'd position and often is sodifficult to reset that it is usually replaced, not only at greater costbut also with an attendant interference with the undamaged brakecircuit. It has therefore been recognized or desired that the warningswitch unit should automatically reset itself upon a return ofa balancedpressure condition after repair of the faulty circuit.

SUMMARY OF THE INVENTION The invention provides improvements in a brakefailure warning device or other differential pressure apparatus which,after signaling a failure in one fluid circuit of a dual system isadapted to proof test a repair of the failure before it may beautomatically reset to a normal condition.

According to the invention, the device may be conveniently arranged toremain in a failure signal condition until relatively high fluidpressure are sustained in the associated fluid circuits. The arrangementmay require proof pressures sufficiently great to test or prove thequality of a repair before the failure signal may be automaticallyeliminated. This feature insures against improper and insufficientrepairs made through either negligence or mistake.

The pressure sensing or warning device includes a pair of fluid pressurechambers each adapted to receive pressure signals from one of thecircuits of a dual system. Pressure responsive means including pressurereaction areas communicating with the fluid chambers assumes a failuresignal condition when a difference in pressure in the chambers reaches acertain value or range. The reset or proof pressure required forresetting the device to a normal'signal condition is a function of therelative size of the pressure reaction areas of the fluid receivingchambers. According to the invention, there is no limitation on therelative sizes of the pressure reaction areas so that a suitablerelationship between such areas may be used to determine the optimumreset pressure for any particular system.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a longitudinalcross-sectional view of one embodiment of a pressure differentialsensing device in accordance with the invention.

FIG. 2 is a longitudinal view, partially in section, showing a secondembodiment of a pressure differential sensing device in accordance withthe invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the embodimentillustrated in FIG. 1, a pressure differential sensing device comprises,in its principal parts, a housing 11 having pressure receiving chambers12 and 13 therein at opposite ends, and pressure responsive means 14 inthe form of a piston assembly of telescoped coaxial pistons or spools l6and 17 in its midportion. In reference to the embodiments of either FIG.1 or FIG. 2, the designation hereinbelow of the juxtaposition of anassembly or a component as at the right side or the left side, forinstance, is for the purposes of description and is not to be taken in alimiting or restrictive sense.

The housing 11, as illustrated, may be a generally H- shaped flat blockhaving, at each side, pairs of threaded ports 21, 22 and 23, 24 adaptedto receive connectors or fittings of a type well known to those familiarin the art. The ports 21-24 communicate with central bore portions ofthe housing 11 through transverse or radial holes 26, 27, 28 and 29respectively. The central portion or interior of the housing 11 isformed with a longitudinal bore having portions 31, 32 and 33 ofdiffering diameters for the reason set forth more fully hereinafter.

At the right end of the housing 11, the bore 33 is closed off by athreaded plug 36 and sealing gasket 37 to define the fluid pressurereceiving chamber 13. Similarly, at the left end, the bore 31 is closedoff by a threaded plug 38 and a sealing washer or gasket 39 to definethe fluid pressure receiving chamber 12. An upset projection 41 on thelower side of the housing 11 secures a bracket 42 to the housing. Thebracket 42 may be used to conveniently mount the device 10, for example,on the body or chassis of an automobile or other vehicle.

The piston 16 is an elongated member slidably disposed in the midportion32 of the axial housing bore. A cylindrical section 46 of the piston 16has a diameter slightly less than the diameter of the mid-bore portion32 and is provided with a peripheral groove 47 therein for the receptionof an O-ring 48. The annular O-ring member 48 and the various otherO-rings described below are of a construction familiar to those skilledin the art and comprise a rubber or elastomeric or other resilientmaterial adapted to sealingly engage, in a known manner, associatedsurrounding wall surfaces defining their working inside and outsidediameters. Where necessary or desirable, circular sealing rings ormembers having a cross section other than that of the illustratedO-rings may be employed.

At its leftward end, the piston 16 includes a cylindrical section 51having a diameter smaller than the diameter of the midsection 46. Thesmaller cylindrical section 51 also includes a groove 52 for carrying aresilient seal or O-ring 53 therein. Near its axial center, the smallercylindrical section 51 is relieved to form a detent or cam groove 54. Asillustrated, the detent 54 may have a hyperbolic profile so that itsdiameter increases along either axial direction from its axial center.

The other piston 17 is a tubular member and includes an outercylindrical surface 56 slightly smaller than the diameter of theassociated bore portion 31 to permit the piston 17 to slide therein. Thepiston 17 includes a peripheral groove 57 in which a resilient O-ring 58is provided to sealingly engage, with its outer diameter, the wall ofthe bore 31. Internally, the piston 17 has a cylindrical bore 61slightly larger in diameter than that of the associated section 51 ofthe elongated piston 16 to allow the pistons 16 and 17 to telescope. Theoutside diameter of O-ring 53 sealingly engages the cylindrical bore 61of the piston 17. At its left, the piston 17 has an axial passage 62extending from the pressure receiving chamber 12 to an end face 64 ofthe elongated pis ton 16. At the juncture of the leftward axial bore 31and midbore 32 an annular generally radial surface 66 provides anabutment or stop against which a rightward or inner end 67 of thetubular piston 17 may bottom.

The end face 64 of the elongated piston l6 is adapted to bottom againsta generally radial surface 68' at the juncture of the bore 61 andpassage 62 of the tubular piston 17. Leftward motion of the elongatedpiston 16 is limited by the bottoming of an enlarged end portion 71 ofthe piston against a radial face 72 at the junction between therightward housing bore 33 and the midhousing bore 32. Rightward motionof the piston 16 is limited by engagement of the enlarged cylindricalportion 71 with the threaded plug 36.

The position of the pistons 16 and 17 shown in FIG. 1 illustrate thenormal condition of the device 10. By inspection and the discussionbelow, it may be understood that both the pistons 16 and 17 may becaused to move leftward in unison, or the elongated piston 16 may becaused to move towards the right. In either case, movement of theelongated piston 16 causes a pin or probe 76 to be moved radiallyoutward from the housing bore 32. The pin 76 projects into the housingbore 32 through a radial hole 77 in the housing. Movement of the pin 76is caused by camming action on the surface of the detent 54 as thepiston 16 moves axially. The pin 76 may be arranged to actuate anelectrical switch or other signal device in a threaded fitting 78mounted on the housing 11. Movement of the pin 76 radially outward isthus adapted to signal the condition or position of the pistons 16 and17.

A vehicle in which the device may be employed as a brake failure warningswitch, generally, will be provided with dual or tandem master hydraulicbrake cylinders each controlling a set of brake slave cylinders at thebrakes of the vehicle in a well known manner. The fluid circuitassociated with one master cylinder portion may be connected with one ofthe fluid receiving chambers 12 or 13 while the circuit associated withthe other master cylinder portion is connected with the other chamber 13or 12 at the ports 21, 22, 23 and 24.

In its illustrated form, the housing 11 provides two ports 21 and 22 atits rightward side and two ports 23 and 24 at its leftward side. Thisarrangement is not necessary for the operation of the device but ratheris a form convenient for use in a vehicle wherein it is desirable tomake use of the device 10 as a terminal block for connecting fluidlines. It will be understood that it is only necessary to provide asingle port at each chamber 12 and 13. v I

Commonly, dual braking systems are arranged to operate withsubstantially equal pressures in each circuit. With such a system, undernormal circumstances when a vehicles brakes are applied the fluidpressures in the opposed chambers 31 and 33, then, will be approximatelyequal. Accordingly, in the embodiments of FIGS. 1 and 2, the proportionsof piston areas in each device are selected for use in a normallyequalized pressure circuit. It may be understood, nevertheless, that theprinciples of the invention may be applied in systems where circuitpressures are generally not equal as occurs when the dual or tandemmaster cylinders have different diameters. If a rupture or other type offailure occurs within one of the circuits, the failed circuit will begenerally incapable of maintaining braking pressure in its associatedchamber 31 or 33 and will result in a pressure imbalance in the pressuresensing devlce.

At the left fluid chamber 12, the pistons 16 and 17, together, generallydefine a major circular piston or pressure reaction area equal to thecross-sectional area of the housing bore 31. A minor circular piston orpressure reaction area at the left chamber 12 is generally defined bythe adjacent cylindrical portion 51 of the elongated piston 16 and isequal to the cross-sectional area of the cylindrical bore 61 of thetubular piston 17. At the other end of the housing 11, a piston orpressure reaction area generally defined by the cross-sectional area ofthe midsection 46 of the piston 16 and equal to the cross-sectional areaof the housing bore 32 communicates with the fluid chamber 13. Thediameter of the housing bore 31, as indicated, is slightly larger thanthe diameter of the mid-housing bore 32 while the internal bore 61 ofthe tubular piston 17 is smaller than the midhousing bore 32.

Under a balanced pressure situation the resultant forces on the pistons16 and 17 will be proportional to their pressure reaction areas. Sincethe major reaction area is defined by the pistons 16 and 17 at the leftchamber 12, the resultant force on the piston assembly 14 is towards theright. Motion of the tubular piston 17, though, is limited by theabutment surface 66 so that the piston assembly 14 will normally remainin the position illustrated in FIG. 1.

If a failure in a fluid circuit connected to the righthand chamber 13occurs so that the pressure in this chamber is substantially less thanthat in the opposite chamber 12, the elongated piston 16 is driven tothe right by fluid pressure operating on the minor pressure reactionarea defined by the cylindrical section 51. The elongated piston 16 willmove rightwardly until the enlarged portion 71 engages the threaded plug36. During this rightward movement, the pin 76 will be cammed upwardlyby the surface of the detent 54 until the pin 76 eventually rests on thecylindrical section 51. Thus, pressure loss through failure in a circuitconnected to the right chamber 13 will be indicated by signal meansoperated by the pin 76. Conveniently, the signal may be the energizationof an electrical warning lamp positioned in view of the operator of thevehicle.

Alternatively, failure in a fluid circuit connected to the lefthandfluid chamber 12 resulting in a loss of pressure therein will cause thepistons 16 and 17 to move leftwardly under the hydraulic force developedby pressure in the righthand chamber 13 against the area of themidsection 46. Motion of the elongated piston 16 to the left will,likewise, cause the pin 76 to be cammed upwardly along the surface ofthe detent 54 until the adjacent cylindrical surface 51 supports the pin76. Leftward motion of the elongated piston 16 is limited when theenlarged cylindrical portion 71 abuts the radial surface 72 at theleftward end of the fluid chamber 13. Motion of the pin 76, again,generates or actuates a warning signal that a failure in a fluid circuithas occurred. It may be appreciated that the elongated piston 16 willremain in a displaced position either to the left or right of thatillustrated, depending on which chamber 12 or 13 is connected to thefailed circuit. At either displaced position, the elongated piston 16will retain the pin 76 in a failure signal condition on a portion of thecylindrical surface 51.

It will be understood that the pistons 16 and 17 are arranged toautomatically return to their illustrated centered or normal conditiononce the failure has been corrected and pressure is reapplied in thechambers 12 and 13. The pin 76 may be spring loaded in a downwardposition to return into the detent 54 and eliminate or deactuate thefailure signal upon recentering of the piston 16.

Assuming the elongated piston 16 was displaced towards the right-handchamber 13 because ofa failure in its associated circuit, upon repair ofthe failure and the application of substantially equal pressures in bothof the fluid chambers 12 and 13 the net fluid force on the elongatedpiston 16 would be to the left to return it to the centered position.This results because the pressure reaction area associated with the leftchamber 12, namely, that defined by the cylindrical section 51 is lessthan the reaction area defined by the larger cylindrical section 46.When the pressure in the chambers 12 and 13 is sufficiently high,friction between the 0- rings 52 and 48 and the surfaces which theysealingly engage and between the pin 76 and the cylindrical section 51will be overcome and the elongated piston 16 will move leftward to thecentered position.

Similarly, where the pistons 16 and 17 have moved to the left upon afailure of the fluid circuit associated with the left chamber 12, thepistons will be recentered upon a repair and application of equalpressures in both of the chambers 12 and 13. The net force on theelongated piston '16 is proportional to the difference between the majorpressure reaction area defined by the pistons 16 and 17 at the leftchamber 12 and the pressure reaction area defined by the elongatedpiston 16 associated with the right chamber 13. When the pressure in thechambers 12 and 13 acting against this difference in area issufficiently high to overcome the friction of the O-rings 58 and 48against their associated bores 31 and 32, respectively, and the frictionof the pin 76 on the cylindrical section 51, the pistons 16 and 17 willbe caused to move rightwardly into the normal condition. A small passage79 is provided in the housing 11 to atmospherically vent the centralportion of the bore 32 and thereby avoid a possible inoperativecondition or hydraulic lock between the pistons 16 and 17 in the eventof seal leakage into this bore 32.

The differences or relative size of the pressure reaction areasdetermined by the cylindrical bores 32, 31, and 61 are chosen withregard to the frictional forces generated by the O-rings 48, 53, 58, andthe pin 76 resisting movement of the pistons 16 and 17. To return thepiston 16 from the right, the difference in area between the area of thecylindrical inner bore 61 of the tubular piston 17 or minor pressurereaction area and the area of the cylindrical bore 32 must be largeenough in relation to a desired proof or reset pressure to overcome thefriction developed by the O-rings 48, 53 and the pin 76 resistingmovement of the piston. Similarly, to return the pistons 16 and 17 fromthe left, the difference in area betwee'n the area of the cylindricalbore 32 and the area of the larger bore 31 or major pressure reactionarea must be large enough in relation to the desired proof or resetpressure to overcome the friction developed by the O-rings 48 and 58 andthe pin 76.

A relatively small area differential will require relatively highpressure levels for recentering. It may be appreciated that, inaccordance with the invention, there is no restriction imposed by thephysical arrangement of the pistons or O-rings, as to how small thesedifferences in area may be made so that the pressure required to resetthe device automatically may be made sufficiently high that it tests thequality of a repair.

In particular, the arrangement of the-pistons 16 and 17 allows thedifference in area between the major and minor pressure reaction areasto be smaller than the cross-sectional area of the O-ring 58, defined bythe inside and outside diameters of the O-ring, since the insidediameter of the O-ring is not restricted by the diameter of the minorpressure reaction area and, as shown, is smaller than the latter.

FIG. 2 illustrates another embodiment of the invention having similarfeatures to the above described embodiment of FIG; 1. As above, adifferential pressure sensing device comprises a housing 111, opposedpressure receiving chambers 112 and 113, and pressure responsive means114. The pressure responsive means 114 comprises a coaxial pistonassembly including a central piston 116 and a pair of substantiallyidentical outer tubular pistons 117. In all essential aspects, thecomponents of the device are symmetrical about a plane midway betweenthe fluid chambers 111 and 113 and perpendicular to the plane of thedrawing.

The housing 111 may be an H-shaped extrusion or block having flat sidesparallel to the plane of the drawing. Threaded ports adapted to beconnected to fluid circuits communicate with radial holes 121 which leadto the pressure receiving chambers 112 and 113. As in the embodiment ofFIG. 1, only one hole 121 is necessary for each chamber 1 12 or 113. Atthe interior of the housing 1 11, there is provided a pair of coaxiallongitudinal bores 126 of equal diameter. The longitudinal bores 126 areclosed off at their outer ends by a pair of threaded plugs 127 to definethe fluid pressure receiving chambers 112 and 113. At their inner ends,the longitudinal bores 126 terminate in generally radial abutmentsurfaces 128. A circular clearance hole 129 runs between these abutmentsurfaces 128 allowing the central piston 116 to extend therethrough.

The tubular pistons 117 are cylindrical in form and each includes anouter cylindrical surface 131, an inner cylindrical bore 132, and acircular fluid passage 133. On its outer periphery, each tubular piston117 is provided with an annular groove 136 carrying an O-ring 137 whichseals at its outer diameter against the asso ciated housing bore 126.The diameter of the outer cylindrical surface 131 of the tubular pistons117 is slightly smaller than the diameter of the housing bores 126 topermit the pistons to slide in these bores.

The central piston 116, preferably, has an outer cylindrical surface 139interrupted near each end of the piston by an annular groove 141 andcentrally by an annular detent 143. The outer cylindrical surface 139has a diameter slightly less than the diameter of the inner bores 132 ofthe tubular pistons 131 to permit the central piston 116 to telescopefreely with the tubular pistons 117. A face 142 at each end of thepiston 116 abuts a generally radial surface 145 of the tubular pistons117 at the base of the inner bores 132 when the pistons are in theillustrated centered position. An ring 144 is provided in each of theannular grooves 141 to seal, at its outer diameter, against thesurrounding inner bore 132 of an associated piston 117.

A pin or probe 151 extends through a radial hole 152 in the housing 111and signals movement of the central piston 116 through a set ofelectrical contacts or other signal means in a fitting 153 threaded intothe housing 111. Rightward or leftward movement of the central piston116 from the position illustrated in FIG. 2, causes the pin 151 to becammed radially upward along the surface of the detent 143 until the pinrests on a portion of the cylindrical surface 139 of the piston.

The device 110 illustrated in FIG. 2 is adapted to sense a pressureimbalance in the chambers 112 and 113 in a manner similar to thatdescribed in connection with the embodiment of FIG. 1. When fluidpressure in the chambers 112 and 113 are substantially equal, thepistons 116 and 117 will remain in the positions illustrated in H6. 2since the pressures react on equal areas of the pistons at each chamber.

Upon a failure in a fluid circuit connected to one of the chambers 112or 113, fluid pressure in the opposite chamber operating against thecentral piston 116 through the passage 133 of the tubular piston 117causes the central piston 116 and the tubular piston 117 associated withthe depressurized chamber 112 to move towards the latter chamber untilthe tubular piston abuts the adjacent threaded plug 127. At this point,the pin 151 rests on the cylindrical surface 139 of the central piston116 at one side of the detent 143 and will remain in this state untilfluid pressure is returned to the depressurized chamber.

After a repair ofa failed circuit, return to a balanced pressurecondition in both of the chambers 112 and 113 will cause the displacedtubular piston 117 and the central piston 116 to return to their normalconditions illustrated in FIG. 2. Renewed pressure in the depressurizedchamber 112 or 113 operates on a major pressure reaction area, definedby the cross-sectional area of the housing bore Y126 and is resisted bythe fluid force in the opposite chamber 113 or 112 operating against aminor pressure reaction area defined by the interior bore 132 of theassociated tubular piston 117. It should be understood that the tubularpiston 117 not associated with the depressurized chamber remains incontact with its adjacent abutment surface 128 so that the fluid forceon this tubular piston is not transmitted to the central piston 116while it returns from its displaced position. A small passage or hole155 is pro-" vided in the housing 111 to atmospherically vent the 1 holeor bore 129 and thereby avoid a possible inoperative condition orhydraulic lock between the tubular pistons 117 in the event of sealleakage into this bore 129.

It may thus be appreciated that the net or resultant fluid force on thecentral piston 116 tending to return it to its normal or centeredcondition is proportional to the difference between the major pressurereaction area, defined by the cross-sectional area of the housing bore126, and the minor pressure reaction area defined by the internal bore132 of the tubular pistons 117. This hydraulic force must overcome thefriction between the O-ring 137 of the displaced tubular piston 117 andthe housing bore 126, the friction between the O-ring 144 and theinternal bore 132 of the undisplaced tubular piston 117, and thefriction of the pin 151 on the outer surfaces of the central piston 116.According to the invention, this differential area between the major andminor pressure reaction areas may be selected, by properly proportioningthe various elements of the device so that a substantial pressure in thechambers 112 and 1 13 is required to overcome the frictional resistance.With a proper selection of differential area, the integrity of a repairin a failed circuit may be made automatically before the pistons 116 and117 return to their normal condition. In this embodiment, as in theembodiment of FIG. 1, the arrangement of the pistons 116 and 117 allowsthe difference in area between the major zlnd minor pressure reactionareas to be smaller than the cross-sectional area of the O-ring 137,defined by the inside and outside diameter of the O-ring since theinside diameter of the O-ring is not restricted by the diameter of theminor pressure reaction area and, as shown, is smaller than the latter.

Although preferred embodiments of this invention have been illustrated,it is to be understood that various modifications and rearrangements ofparts may be resorted to without departing from the scope of theinvention disclosed and claimed herein.

What is claimed is:

l. A differential pressure sensing device comprising a housing, firstand second fluid pressure receiving chambers each adapted to beconnected to a separate source of fluid pressure, pressure responsivemeans in said housing, said pressure responsive means providing at saidfirst chamber major and minor circular pressure reaction areas definedby major and minor piston diameters, a first annular sealing membersealing the periphery of the major circular reaction area of the majorpiston diameter, a second annular sealing member sealing the peripheryof the minor circular'reaction area of the minor piston diameter, saidfirst and second sealing members each being independent in size relativeto the respective piston diameter of the other, said pressure responsivemeans providing at said second chamber a pressure reaction area having apredetermined relation to the pressure reaction areas associated withthe first chamber, said pressure responsive means being sensitivethrough said reaction areas to fluid pressures in said chambers, saidpressure responsive means including means to actuate a signal detectableexternally of said housing and operable when the pressure in saidchambers change substantially from a normal relationship therebetweenand to self-maintain said signal after the substantial change inpressure relationship is eliminated, said pressure responsive meansincluding means to deactuate said signal, said signal deactuating meansrequiring predetermined reset pressures having magnitudes generallyinversely proportional to the difference in size of the sealingdiameters of said first and second sealing members supplied at saidchambers.

2. In a brake failure warning device for use in a vehicle having dualfluid brake circuits including a housing, a pair of fluid pressurereceiving chambers in said housing, means to connect each of the fluidcircuits to one of said chambers, a passage in said housing extendingbetween said chambers, a pair of coaxial pistons in said passage andcommunicating with a first one of said chambers, one of said pistonsdefining a circular major pressure reaction area and the other pistondefining a minor circular pressure reaction area smaller than said majorarea, said pistons each defining a normally centered position in saidhousing, said major area piston being limited to axial movement fromsaid centered position towards said first chamber and back to saidcentered position, said minor area piston being axially reciprocaltowards and away from both of said chambers, piston means associatedwith a second one of said chambers causing pressure in the secondchamber to force said pistons towards said first chamber upon loss offluid pressure in said first chamber, means to sense movement of saidpistons and produce a warning signal externally of said housing, renewalof pressure in said first chamber causing a return of said pistons awayfrom said first chamber to said centered position, said piston meansresisting return movement of said pistons from said first chambertowards said centered position, said piston means providing a pressurereaction area at said second chamber of a predetermined size in relationto 'the normal pressure in said second chamber such that thepressure-area product therein is not less than the product of said minorarea and the normal pressure in said first chamber and is not greaterthan the product of the major area and the normal pressure in said firstchamber, the improvement comprising separate sealing means associatedwith said major and minor area pistons including sealing ring meanspermitting selection of the size differential of said major and minorareas substantially independent of the configuration of said sealingring means whereby said size differential may be selected sufficientlysmall to require substantial pressures to be introduced into saidchambers before said pistons are caused to return to their centeredcondition from said first chamber.

3. A brake warning device as set forth in claim 2, wherein said sealingring means is associated with said minor area piston and said ring meanshas an outer diameter equal to the diameter of said minor area.

4. A brake warning device as set forth in claim 3, wherein said sealingmeans includes a second sealing ring associated with said major areapiston.

5. A brake warning device as set forth in claim 2, wherein said pistonmeans associated with said second chamber provides a pressure reactionarea having a size intermediate to that of said minor area and saidmajor area.

6. A brake warning device as set forth in claim 2, wherein said pistonmeans of said second chamber comprises a pair of coaxial pistonsproviding major and minor pressure reaction areas substantially the samein size as said major and minor areas associated with said firstchamber.

7. A differential pressure sensing device comprising a housing, firstand second fluid pressure receiving chambers each adapted to beconnected to a separate source of fluid pressure, pressure responsivemeans in said housing, said pressure responsive means providing at saidfirst chamber major and minor circular pressure reaction areas definedby major and minor piston diameters, first and second separate means forsealing said major and minor piston diameters respectively, said firstand second means for sealing each being independentin size relative tothe respective piston diameter of the other, said pressure responsivemeans providing at said second chamber a pressure reaction area having apredetermined relation to the pressure reaction areas associated withthe first chamber, said pressure responsive means being sensitivethrough said reaction areas to fluid pressures in said chambers, saidpressure responsive means including means to actuate a signal detectableexternally of said housing and operable when the pressuresyin saidchambers change substantially from a normal relationship therebetweenand to selfmaintain said signal after the substantial change in pressurerelationship is eliminated, said pressure responsive means includingmeans to deactuate said signal, said signal deactuating means requiringpredetermined reset pressures having magnitudes generally inverselyproportional to the difference in size of the sealing diameters of saidfirst and-second means for sealing supplied at said chambers.

1. A differentIal pressure sensing device comprising a housing, firstand second fluid pressure receiving chambers each adapted to beconnected to a separate source of fluid pressure, pressure responsivemeans in said housing, said pressure responsive means providing at saidfirst chamber major and minor circular pressure reaction areas definedby major and minor piston diameters, a first annular sealing membersealing the periphery of the major circular reaction area of the majorpiston diameter, a second annular sealing member sealing the peripheryof the minor circular reaction area of the minor piston diameter, saidfirst and second sealing members each being independent in size relativeto the respective piston diameter of the other, said pressure responsivemeans providing at said second chamber a pressure reaction area having apredetermined relation to the pressure reaction areas associated withthe first chamber, said pressure responsive means being sensitivethrough said reaction areas to fluid pressures in said chambers, saidpressure responsive means including means to actuate a signal detectableexternally of said housing and operable when the pressure in saidchambers change substantially from a normal relationship therebetweenand to self-maintain said signal after the substantial change inpressure relationship is eliminated, said pressure responsive meansincluding means to deactuate said signal, said signal deactuating meansrequiring predetermined reset pressures having magnitudes generallyinversely proportional to the difference in size of the sealingdiameters of said first and second sealing members supplied at saidchambers.
 2. In a brake failure warning device for use in a vehiclehaving dual fluid brake circuits including a housing, a pair of fluidpressure receiving chambers in said housing, means to connect each ofthe fluid circuits to one of said chambers, a passage in said housingextending between said chambers, a pair of coaxial pistons in saidpassage and communicating with a first one of said chambers, one of saidpistons defining a circular major pressure reaction area and the otherpiston defining a minor circular pressure reaction area smaller thansaid major area, said pistons each defining a normally centered positionin said housing, said major area piston being limited to axial movementfrom said centered position towards said first chamber and back to saidcentered position, said minor area piston being axially reciprocaltowards and away from both of said chambers, piston means associatedwith a second one of said chambers causing pressure in the secondchamber to force said pistons towards said first chamber upon loss offluid pressure in said first chamber, means to sense movement of saidpistons and produce a warning signal externally of said housing, renewalof pressure in said first chamber causing a return of said pistons awayfrom said first chamber to said centered position, said piston meansresisting return movement of said pistons from said first chambertowards said centered position, said piston means providing a pressurereaction area at said second chamber of a predetermined size in relationto the normal pressure in said second chamber such that thepressure-area product therein is not less than the product of said minorarea and the normal pressure in said first chamber and is not greaterthan the product of the major area and the normal pressure in said firstchamber, the improvement comprising separate sealing means associatedwith said major and minor area pistons including sealing ring meanspermitting selection of the size differential of said major and minorareas substantially independent of the configuration of said sealingring means whereby said size differential may be selected sufficientlysmall to require substantial pressures to be introduced into saidchambers before said pistons are caused to return to their centeredcondition from said first chamber.
 3. A brake warning device as setforth in claim 2, wherein said sealing ring means is associated withsaid minor area piston and said ring means has an outer diameter equalto the diameter of said minor area.
 4. A brake warning device as setforth in claim 3, wherein said sealing means includes a second sealingring associated with said major area piston.
 5. A brake warning deviceas set forth in claim 2, wherein said piston means associated with saidsecond chamber provides a pressure reaction area having a sizeintermediate to that of said minor area and said major area.
 6. A brakewarning device as set forth in claim 2, wherein said piston means ofsaid second chamber comprises a pair of coaxial pistons providing majorand minor pressure reaction areas substantially the same in size as saidmajor and minor areas associated with said first chamber.
 7. Adifferential pressure sensing device comprising a housing, first andsecond fluid pressure receiving chambers each adapted to be connected toa separate source of fluid pressure, pressure responsive means in saidhousing, said pressure responsive means providing at said first chambermajor and minor circular pressure reaction areas defined by major andminor piston diameters, first and second separate means for sealing saidmajor and minor piston diameters respectively, said first and secondmeans for sealing each being independent in size relative to therespective piston diameter of the other, said pressure responsive meansproviding at said second chamber a pressure reaction area having apredetermined relation to the pressure reaction areas associated withthe first chamber, said pressure responsive means being sensitivethrough said reaction areas to fluid pressures in said chambers, saidpressure responsive means including means to actuate a signal detectableexternally of said housing and operable when the pressures in saidchambers change substantially from a normal relationship therebetweenand to self-maintain said signal after the substantial change inpressure relationship is eliminated, said pressure responsive meansincluding means to deactuate said signal, said signal deactuating meansrequiring predetermined reset pressures having magnitudes generallyinversely proportional to the difference in size of the sealingdiameters of said first and second means for sealing supplied at saidchambers.